States of matter Revision Notes

In solids, particles are tightly packed in a regular, fixed arrangement. The particles vibrate about fixed positions but do not move freely. This close packing results in a definite shape and volume for solids.

The strong forces of attraction between particles keep them in fixed positions, preventing flow or compression under normal conditions.

• Particles are closely packed in a fixed, regular pattern.
• Particles vibrate but do not change position.
• Strong intermolecular forces hold particles together.
• Solids have a definite shape and volume.

In liquids, particles are close together but not in a fixed arrangement. They can move past each other, allowing liquids to flow and take the shape of their container.

The forces of attraction are weaker than in solids but still significant, so liquids have a definite volume but no fixed shape.

Because particles can move, liquids are less rigid and can be poured.

• Particles are close but arranged randomly.
• Particles move by sliding past each other.
• Intermolecular forces are weaker than in solids but still present.
• Liquids have a definite volume but take the shape of their container.

Gas particles are widely spaced with large distances between them compared to their size. They move rapidly and randomly in all directions.

The forces of attraction between particles are negligible, so gases have neither a definite shape nor volume.

Gases expand to fill any container and are easily compressed due to the large spaces between particles.

• Particles are far apart and randomly arranged.
• Particles move quickly in all directions.
• Intermolecular forces are negligible.
• Gases have no fixed shape or volume and are compressible.

The particle arrangements explain the macroscopic properties of each state: solids maintain shape and volume; liquids maintain volume but adapt shape; gases adapt both shape and volume.

Compressibility differs: solids and liquids are nearly incompressible due to close packing, while gases are highly compressible.

Density is highest in solids, lower in liquids, and lowest in gases because of particle spacing.

• Definite shape and volume in solids due to fixed particles.
• Liquids flow and take container shape but keep volume.
• Gases fill containers completely and are compressible.
• Density decreases from solids to gases.
• Compressibility is negligible in solids and liquids, significant in gases.

To represent states of matter, draw particles as circles:

For solids: draw closely packed circles in a regular grid with no gaps.

For liquids: draw circles close together but arranged irregularly with small gaps, showing some movement space between particles (arrows optional). For gases: draw circles spaced widely apart randomly, with arrows indicating rapid, random motion in all directions.

• Solid diagram: regular, tight packing, no particle movement shown.
• Liquid diagram: irregular close packing, some gaps, particles can move past each other.
• Gas diagram: widely spaced particles, random directions of motion indicated by arrows.

Matter commonly exists in three states: solid, liquid, and gas. Each state has distinct particle arrangements and behaviours. Transitions between these states are called phase changes or state changes.

Phase changes occur when matter absorbs or releases energy, usually in the form of heat, causing particles to rearrange and change their movement patterns.

The main phase changes between solids, liquids, and gases are melting, freezing, evaporation, and condensation. This note excludes sublimation and deposition (gas to solid and solid to gas transitions).

• Solid: particles are closely packed in a fixed, regular arrangement and vibrate about fixed positions.
• Liquid: particles are close together but can move past each other, allowing flow.
• Gas: particles are far apart and move freely at high speeds.

Melting is the process where a solid changes into a liquid when it is heated to its melting point.

During melting, the solid absorbs thermal energy (latent heat of fusion) without a temperature increase until the entire solid has melted.

The added energy breaks some of the forces holding particles in fixed positions, allowing them to move more freely as a liquid.

• Melting point is the temperature at which a solid turns into a liquid.
• Energy absorbed during melting is called latent heat of fusion, measured in joules (J).
• Particle arrangement changes from fixed lattice to a less ordered, mobile arrangement.

Freezing is the reverse of melting, where a liquid loses heat and changes into a solid at its freezing point, which is the same temperature as the melting point for pure substances.

During freezing, the liquid releases latent heat of fusion as particles slow down and arrange into a fixed, regular pattern.

Temperature remains constant during the phase change until the entire liquid has solidified.

• Freezing point equals melting point for pure substances.
• Latent heat of fusion is released to the surroundings during freezing.
• Particles become fixed in position, forming a solid lattice.

Evaporation is the process where particles at the surface of a liquid gain enough energy to escape into the gas phase.

It can occur at any temperature but increases with temperature, surface area, and air movement.

Evaporation causes cooling because higher-energy particles leave the liquid, lowering the average kinetic energy of remaining particles.

• Evaporation is a surface phenomenon.
• Energy absorbed during evaporation is called latent heat of vaporisation, measured in joules (J).
• Particle arrangement changes from close contact to widely spaced and fast-moving gas particles.

Condensation is the process where gas particles lose energy and change into a liquid.

It occurs when gas is cooled or compressed, causing particles to slow down and come closer together.

Latent heat of vaporisation is released during condensation, warming the surroundings.

• Condensation occurs at the boiling point or below when gas cools.
• Energy is released to the surroundings as latent heat of vaporisation.
• Particles move from free, fast motion to closer, slower movement in liquid form.